Apparatus providing simplified alignment of optical fiber in photonic integrated circuits

ABSTRACT

A structure for optically aligning an optical fiber to a photonic device and method of fabrication of same. The structure optically aligns an optical fiber to the photonic device using a lens between the two which is moveable by actuator heads. The lens is moveable by respective motive sources associated with the actuator heads.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. application Ser. No.15/640,041, filed Jun. 30, 2017; which is a continuation of U.S.application Ser. No. 15/362,578, filed Nov. 28, 2016, now U.S. Pat. No.9,715,070; which is a continuation of U.S. application Ser. No.15/134,167, filed Apr. 20, 2016, now U.S. Pat. No. 9,507,104; which is adivisional of U.S. application Ser. No. 13/732,557, filed Jan. 2, 2013,now U.S. Pat. No. 9,341,787; each of which is incorporated herein byreference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with Government support under Contract Number9999404-12-0004 awarded by DARPA. The Government has certain rights inthe invention.

TECHNICAL FIELD

Embodiments of the invention provide alignment structures for opticallyaligning an optical fiber to a photonic device in photonic integratedcircuits.

BACKGROUND

Optical signal transmission may be used to communicate signals betweenseparated integrated circuit chips to provide inter-chip connections andwithin components on the same integrated circuit chip to provideinter-chip connections. In many instances it is necessary to couple anexternal optical fiber to a photonic device, e.g., a waveguide, of anintegrated circuit photonics chip. Such coupling requires preciseoptical alignment between the optical fiber and the photonic device.

In order to achieve such precise optical alignment, there have beenproposed complex microelectromechanical systems (MEMS) which are builton an integrated circuit chip for optically aligning an optical fiberand photonic device. Complex MEMS structures, however, are expensive toimplement and time-consuming to fabricate. In addition, a structure foraligning an optical fiber to a photonic device should account foralignment changes which may occur during use, for example, due totemperature changes or other perturbating influences which may occur.What is needed is a simple structure for optically aligning andmaintaining optical alignment of an optical fiber to a photonic deviceon an integrated circuit chip.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an embodiment of an optical alignmentstructure;

FIG. 2 is a cross-sectional view showing greater detail of a portion ofthe FIG. 1 embodiment;

FIG. 3 is another cross-sectional view showing greater detail of aportion of the FIG. 1 embodiment;

FIG. 4 is an enlargement of a portion of FIG. 1;

FIGS. 5A through 5F show in cross-sectional view operational movementsof actuator heads depicted in FIG. 1;

FIG. 6 is a cross-sectional view of another embodiment;

FIG. 7 is a cross-sectional view of another embodiment;

FIG. 8 is a cross-sectional view of another embodiment;

FIG. 9 is a cross-sectional view of another embodiment;

FIG. 10 is a cross-sectional view of another embodiment;

FIG. 11 is a perspective view of a signal detector;

FIG. 12 is a top view of another signal detector; and,

FIG. 13 illustrates a control system which can be used to dynamicallyoptically align an optical fiber with a photonic device.

DETAILED DESCRIPTION

Embodiments described herein provide a simplified structure foroptically aligning an optical fiber with a photonic device, the latterbeing provided on an integrated circuit photonics chip which containsoptical devices for propagating and processing light signals. Thestructure can be fabricated using known MEMS techniques on asemiconductor, e.g., silicon, substrate of an integrated circuitphotonics chip. An alignment lens is provided between the optical fiberand photonic device. The lens is moveable in three directions (x, y andz) to produce and maintain optical alignment of the optical fiber andphotonic device. A pair of lower actuator heads hold the lens from belowand are respectively provided at the free ends of a pair of cantileverarms. Another upper actuator head is provided at the free end of anothercantilever arm and holds the lens from above and in place on the lowerpair of actuator heads. A respective motive source, e.g., apiezoelectric structure, is associated with each of the cantilever arms.The motive source is arranged to bend the arms to move the respectiveactuator heads and thus the lens in the three directions to acquire andmaintain optical alignment between the optical fiber and photonicdevice. Alternatively, the cantilever arms connected to the loweractuators can be non-bendable, or another fixed structure can be used,to support the lower actuator heads with a respective motive sourceprovided between the lower actuator heads and fixed structure to affecttheir movement. In another alternative, motive sources can also beprovided between the lens and fixed actuator heads. A control loop canbe used to acquire a received optical signal and establish and maintainoptical alignment by appropriately activating the motive sources inresponse to the received optical signal to move the actuator heads andhold the lens in a position which optically aligns the optical fiber andphotonic device.

Referring now to the drawings, FIG. 1 shows a perspective view of anembodiment of the invention, while FIGS. 2 and 3 show in cross-sectiondetails of elements of the FIG. 1 embodiment. An integrated circuitphotonics chip 9 formed of a semiconductor, e.g., silicon, substratewhich includes photonic devices, and which may also include electricaldevices and circuits on the same chip, is shown. The chip 9 includes afabricated waveguide 17, having a waveguide core 17 a and associatedcladding 17b. The waveguide 17 receives or transmits an optical signalfrom or to an optical fiber 11 which is adhered by an adhesive at oneend to a cradle 13 fabricated on the integrated circuit chip 9 and whichhas a curved upper surface which accepts the curved outer surface ofoptical fiber 11. Optical signals transmitted between the optical fiber11 and waveguide 17 pass through a lens 15.

The lens 15 may take various forms, including but not limited to, a balllens, a barrel lens and a spherical lens. In the following discussion aball lens is discussed as an example of lens 15. The lens 15 isadjustably held in the optical path between the optical fiber 11 andwaveguide 17 by lower actuator heads 21 a, 21 b which have downwardlyinclined faces 26 a, 26 b (FIG. 2) which engage with a lower surface oflens 15. The lower actuator heads 21 a, 21 b are provided at the freeends of respective cantilever arms 23 a, 23 b. The support ends of thecantilever arms 23 a, 23 b are integrally connected to respective bases20 a, 20 b which are fabricated on integrated circuit chip 9. Thecantilever arms 23 a, 23 b are independently bendable in the x axisdirection as shown in FIGS. 2 and 4, by virtue of respective motivesources which may take the form of respective independent piezoelectricstructures 25 a, 25 b provided along the length of each of thecantilever arms 23 a, 23 b. The piezoelectric structures 251, 25 b maytake the form of a known biomorphic structure which includes a firstlayer of silicon and a second layer of piezoelectric material such asPZT or PMN. The independent bending of the cantilever arms 23 a, 23 bcauses independent movement of the actuator heads 21 a, 21 b in the xaxis direction in accordance with voltages applied to the piezoelectricstructures 25 a, 25 b.

The lens 15 is also held at its upper surface by an upper actuator 21 cwhich may be in the form of a cap or other structure for engaging with aportion of the upper surface of lens 15. The upper actuator 21 c isprovided at the free end of a cantilever arm 23 c. A motive source isprovided along the side of cantilever arm 23 c and can be in the form ofa piezoelectric structure 25 c. The piezoelectric structure 25 c causesbending of cantilever arm 23 c and thus movement of the lens 15 in the yaxis direction, as shown in FIGS. 1 and 3. As described in detail below,the independent movement of the cantilever arms 23 a, 23 b and 23 c bythe corresponding piezoelectric structures 25 a, 25 b, and 25 c enablesthe lens 15 to be moved in the x, y, and z axis directions shown inFIGS. 1-4.

FIG. 2 shows a cross-sectional view along the lines 2-2 of FIG. 1. TheFIG. 2 cross-section is through the lens 15 and lower actuator heads 21a, 21 b. FIG. 3 shows a cross-sectional view through the optical fiber11, lens 15 and waveguide 17 along lines 3-3 of FIG. 1. As shown in FIG.2, each lower actuator head 21 a, 21 b has a sloped face 26 a, 26 b onwhich lens 15 rests. FIG. 2 shows the lower actuator heads 21 a, 21 b ashaving a planar downwardly sloping face 26 a, 26 b; however, asdescribed below, other profiles can be used for the faces of theactuator heads 21 a, 21 b.

FIGS. 5A-5F show how lens 15 is moveable in the x axis and z axisdirections in accordance with movement of the lower actuator heads 21 a,21 b upon actuation by respective piezoelectric structures 25 a, 25 b.To accommodate the z axis movement, cantilever arm 23 c, which holdsactuator head 21 c on the top of lens 15, is bendable up and down. FIG.5A shows by arrow A lens 15 moving upwardly when the piezoelectricstructures 25 a, 25 b are each activated to bend respective cantileverarms 23 a, 23 b and move both actuator heads 21 a, 21 b towards oneanother, while FIG. 5B shows by arrow B lens 15 moving downwardly whenthe piezoelectric structures 25 a, 25 b are each activated to bendrespective cantilever arms 23 a, 23 b and move both the actuator heads21 a, 21 b away from one another. FIGS. 5C and 5D show by respectivearrows C and D movement of lens 15 to the left and right respectivelywhen the piezoelectric structures 25 a, 25 b are each activated to bendrespective cantilever arms 23 a, 23 b and move both actuator heads 21 a,21 b to the left and to the right. FIG. 5E shows by arrow E a leftwardupward movement of lens 15 when actuator 21 b is not operated bypiezoelectric structure 25 b, and actuator 21 a is moved to the left byoperation of piezoelectric structure 25 a and the bending of cantileverarm 23 a. FIG. 5F shows by arrow F movement of lens 15 whenpiezoelectric structure 25 b bends cantilever arm 23 b to move actuatorhead 21 b to the right while actuator head 21 a remains stationary.

As noted, cantilever arm 23 c is bendable in the z axis direction asactuator 21 c moves up and down in response to movement of the lens 15by the actuator heads 21 a. 21 b. In addition, the piezoelectricstructure 25 c associated with cantilever arm 23 c causes arm 23 c tobend in the y axis direction to move actuator 21 c and lens 15 along they axis. As a result, lens 15 is moveable along all three axes x, y andz.

FIG. 6 shows in cross section modified lower actuator heads 21 a′ and 21b′, each provided with respective downwardly inclined stepped surfaces30 a, 30 b for contacting lens 15. The stepped surfaces 30 b, 30 aprovide a stepped incremental adjustment of lens 15 when moving in thedirections indicted by FIGS. 5A, 5B, 5E and 5F. FIG. 7 shows othermodified lower actuator heads 21 a″ and 21 b″ which have respectivecurved actuator surfaces 30 a′ and 30 b′.

Movement of the actuator heads 21 a, 21 b, 21 c in the embodimentsdescribed above occurs by appropriate electrical actuation of respectivepiezoelectric structures 25 a, 25 b, 25 c which bend respectivecantilever arms 23 a, 23 b, 23 c. In another embodiment, the cantileverarms 23 a, 23 b for lower activator heads 21 a, 21 b can be non-bendableand piezoelectric structures 25 a, 25 b omitted from the cantilever arms23 a, 23 b. Instead, as shown in FIG. 8, piezoelectric structures 35 a,35 b can be located between respective non-bendable cantilever arms 23a, 23 b and their associated actuator heads 21 a, 21 b. In thisarrangement, the piezoelectric structures 35 a, 35 b are eachindependently operated to expand and compress as shown by arrows Hthereby moving the respective actuator heads 21 a and 21 b to positionlens 15 in the manner shown in FIGS. 5A-5F. In another alternativeembodiment, as shown in FIG. 9, the cantilever arms 23 a, 23 b can beomitted and the piezoelectric structures 35 a, 35 b attached directly tosupport bases 20 a′ and 20 b′ fabricated on the chip 9. In yet anotherembodiment shown in FIG. 10, the piezoelectric structures 35 a, 35 b canbe formed directly on the surfaces of the lower actuator heads 21 a, 21b where they directly engage with the lens 15 and compress or contact inthe directions of arrows Ito move lens 15.

In order to operate the piezoelectric structure 25 a, 25 b, 25 c or 35a, 35 b, 25 c an alignment control system is provided which samples andmonitors the strength of a received optical signal to control theposition of lens 15. FIG. 11 illustrates a signal detector 29 fabricatedon waveguide core 17 a which can be formed of germanium orgermanium-silicon and which receives an optical signal from waveguidecore 17 a corresponding to an optical signal transmitting to waveguide17 from optical fiber 11. Signal detector 29 outputs an electricalsignal 31 corresponding to the received optical signal. FIG. 12illustrates a signal detector 41 which can also be formed of germaniumor germanium silicon which can be coupled to an optical fiber 11receiving an optical signal from a waveguide 17. Signal detector 41 isevanescently coupled to optical fiber 11 by a waveguide 39 and can alsoprovide an output signal 31. Detectors 29 and 41 can provide outputsignals representing the strength of a received optical signal which canbe used by a control system to control the position of lens 15.

FIG. 13 illustrates one example of a control system 51 which receivesthe signal strength output signals from either detector 29 or detector41 and which provides actuating signals for piezoelectric structures 25a, 25 b, 25 c (or 35 a, 35 b, and 25 c). The control system 51 can beimplemented as a hardware circuit structure integrated on integratedcircuit chip 9 or as software running on a processor structureintegrated on chip integrated circuit 9, or as a combination of the two.For simplification, the control system 51 will be described as theoperational steps which are executed by the circuit structure,programmed processor structure, or combination of the two.

In step 101, an initial central x axis, y axis, and z axis position oflens 15 is set and the piezoelectric structures (25 a, 25 b, 25 c or 35a, 35 b, 25 c) are actuated to obtain this central lens 15 position. Atstep 103 an optical signal strength is acquired from either detector 29or 41, depending on whether waveguide 17 is receiving an optical signalfrom optical fiber 11 or vice versa. In step 105 the signal strength ofthe received optical signal is compared with a reference signal strengthto see if it is within an acceptable tolerance range, e.g., above apre-set amplitude level. If the answer is yes, the current x, y, zposition of lens 15 is held by maintaining the current actuation stateof the piezoelectric structures 25 a, 25 b, 25 c or 35 a, 35 b, 25 c atstep 107. The operational flow then returns to step 103, with or withouta predetermined delay 119, where another signal is acquired from eitherdetector 29 or 41. A predetermined delay, shown as an option by thedotted step 119, will cause a periodic rather than a continuousacquisition of a signal from the detector 29 or 41 after an x, y, zposition is set for lens 15.

If the signal strength acquired from the detector 29 or 41 is not withinan acceptable tolerance range, as determined in step 105, control system51 actuates the piezoelectric structures 25 a, 25 b, 25 c (or 35 a, 35b, 25 c) to change the x, y and z lens position by setting a new x, y, zlens 15 position in step 109. A signal is then acquired in step 111 fromeither detector 29 or 41, and a signal strength at that new x, y, z lensposition is measured and stored in step 113. In step 115 a determinationis made if all possible x, y, z positions of lens 15 have been set andcorresponding signal strength values stored. If the answer is yes, thex, y, z position having the highest stored signal strength is set as theposition of lens 15 and the piezoelectric structures 25 a, 25 b, 25 c(or 35 a, 35 b, 25 c) correspondingly actuated in step 117 to set thelens 15 at that position. Following step 117, the control system 51reverts back to step 103, with or without the predetermined delay 119.

If in step 115 not all x, y, z positions have been set for lens 15 andcorresponding signal strengths stored, the control system 51 returns tostep 109 and the operation implemented by steps 109-115 repeats until ayes condition is detected in step 115.

Thus, control system 51 starts from an initial central x, y, z lens 15position and if the signal strength, which can be checked periodicallyor continuously, is not within an acceptable tolerance range as detectedat step 105, it finds a new x, y, z position for lens 15 where signalstrength is a maximum and sets that as the new x, y, z lens 15 position.The control system 51 operates dynamically to periodically orcontinuously shift lens 15, if adjustment is needed, to set it at thebest position to optionally align the optical fiber 11 and waveguide 17.It should be noted that FIG. 13 represents but one example of a controlprocess which can be implemented by control system 51 and used to adjustlens 15 to a position which best aligns optical signals between opticalfiber 11 and a waveguide 17. Other control processes 51 can also beused.

The structures illustrated herein are fabricated on integrated circuitphotonics chip 9 using known MEMS techniques which fabricate and shapestructures from the substrate material of the chip 9.

While example embodiments have been described in detail, it should bereadily understood that the invention is not limited to the disclosedembodiments. Rather the embodiments can be modified to incorporate anynumber of variations, alterations, substitutions, or equivalentarrangements not heretofore described without departing from the scopeof the invention which is defined solely by the scope of the appendedclaims.

I/We claim:
 1. An optical alignment structure comprising: a lensconfigured to receive an optical signal from an optical emitter and todirect the light signal to an optical receiver; a plurality of actuatorheads configured to engage the lens and to permit motion of the lens inthree mutually-orthogonal directions; a motive source associated withthe plurality of actuator heads for causing movement of the lens.
 2. Theoptical alignment system of claim 1, wherein the plurality of actuatorheads, the motive source, and at least one of the optical emitter or theoptical receiver are part of an integrated photonic chip.
 3. The opticalalignment structure of claim 1, wherein the motive source comprisesfirst, second and third motive sources, each associated with arespective one of the plurality of actuator heads for causing movementof the lens.
 4. The optical alignment structure of claim 1, wherein themotive source comprises a piezoelectric structure for causing movementof at least one of the plurality of actuator heads.
 5. The opticalalignment structure of claim 1, wherein first and second ones of theplurality of actuator heads are each provided with a downwardly slopinginclined area engaging with a lower portion of the lens.
 6. The opticalalignment structure of claim 5, wherein the first and second ones of theplurality of actuator heads are spaced apart and positioned on oppositesides of a bottom of the lens such that the lens engages with theinclined areas of the first and second ones of the plurality of actuatorheads.
 7. The optical alignment structure of claim 6, wherein the firstand second ones of the plurality of actuator heads cause the lens tomove in an upward direction when the first and second ones of theplurality of actuator heads are moved towards one another and cause thelens to move in a downward direction when the first and second ones ofthe plurality of actuator heads are moved away from one another.
 8. Theoptical alignment structure of claim 6, wherein the first and secondones of the plurality of actuator heads cause the lens to move in aleftward direction when the first and second ones of the plurality ofactuator heads move in leftward direction and cause the lens to move ina rightward direction when the first and second ones of the plurality ofactuator heads move in a rightward direction.
 9. The optical alignmentstructure of claim 1, further comprising a control system for operatingthe motive source to cause movement of the plurality of actuator heads.10. The optical alignment structure of claim 9, wherein the controlsystem receives a signal representing the amount of light received bythe optical receiver and operates the motive source to cause movement ofthe lens to a position which obtains optical alignment of the opticalemitter and the optical receiver.
 11. The optical alignment structure ofclaim 10, wherein the control system periodically monitors the lightreceived by the optical receiver and, if the received light is notwithin a tolerance range, operates the motive source to cause movementof the lens to a position which increases the amount of light receivedby the optical receiver.
 12. The optical alignment structure of claim 1,wherein the optical emitter is an optical fiber and the optical receiveris a photonic device.
 13. The optical alignment structure of claim 12,wherein the photonic device is a waveguide.
 14. The optical alignmentstructure of claim 1, wherein the optical emitter is a photonic deviceand the optical receiver is an optical fiber.
 15. The optical alignmentstructure of claim 14, wherein the photonic device is a waveguide. 16.The optical alignment structure of claim 1, wherein the threemutually-orthogonal directions comprise an up-down direction, aleft-right direction, and a forward-back direction.
 17. An opticalalignment structure comprising: a lens configured to receive an opticalsignal from an optical emitter and to direct the light signal to anoptical receiver; a plurality of actuator heads engaging the lens andconfigured to permit motion of the lens in three mutually-orthogonaldirections; a plurality of cantilever arms for respectively supportingthe plurality of actuator heads; a plurality of motive sourcesrespectively associated with each cantilever arm; and a control systemfor controlling the motive sources to effect movement of the lensthrough the actuator heads.
 18. The optical alignment structure of claim17, wherein one of the optical emitter and the optical receivercomprises a photonic device and the other of the optical emitter and theoptical receiver comprises an optical fiber.
 19. The optical alignmentstructure of claim 17, wherein the motive sources comprise piezoelectricstructures.
 20. The optical alignment structure of claim 17, wherein thelens, actuator heads, cantilever arms, motive sources, and at least oneof the optical emitter or the optical receiver are fabricated on acommon integrated circuit chip.
 21. The optical alignment structure ofclaim 20, wherein the control system is fabricated on the commonintegrated circuit chip.
 22. The optical alignment structure of claim17, further comprising a signal detector for receiving a light signalfrom the optical receiver, the control system receiving and acting onthe light signal to control the motive sources.
 23. The opticalalignment structure of claim 17, wherein the optical emitter is anoptical fiber and the optical receiver is a photonic device.
 24. Theoptical alignment structure of claim 23, wherein the photonic device isa waveguide.
 25. The optical alignment structure of claim 17, whereinthe optical emitter is a photonic device and the optical receiver is anoptical fiber.
 26. The optical alignment structure of claim 25, whereinthe photonic device is a waveguide.
 27. The optical alignment structureof claim 17, wherein the three mutually-orthogonal directions comprisean up-down direction, a left-right direction, and a forward-backdirection.